On Wed, Jul 08, 2015 at 12:16:30PM +0200, Jens Wiklander wrote:

+static void tee_device_complete_unused(struct kref *kref)
+{
+	struct tee_device *teedev;
+
+	teedev = container_of(kref, struct tee_device, users);
+	/* When the mutex is released, no other tee_device_get() will succeed */
+	teedev->desc = NULL;
+	complete(&teedev->c_no_users);
+}
+
+void tee_device_put(struct tee_device *teedev)
+{
+	mutex_lock(&teedev->mutex);
+	/* Shouldn't put in this state */
+	if (!WARN_ON(!teedev->desc))
+		kref_put(&teedev->users, tee_device_complete_unused);
+	mutex_unlock(&teedev->mutex);
+}
+
+bool tee_device_get(struct tee_device *teedev)
+{
+	mutex_lock(&teedev->mutex);
+	if (!teedev->desc) {
+		mutex_unlock(&teedev->mutex);
+		return false;
+	}
+	kref_get(&teedev->users);
+	mutex_unlock(&teedev->mutex);
+	return true;
+}

If you are holding the mutex then you don't really need a kref, just a
simple active count counter.

I've been a bit learly lately about seeing krefs used for something
other than kfree, I've seen a few subtle mistakes in those schemes -
yours looks OK, only because of the lock, and the lock makes the kref
redundant..

+       cdev_init(&teedev->cdev, &tee_fops);
+       teedev->cdev.owner = teedesc->owner;

This also needs to set teedev->cdev.kobj.parent.
I'm guessing:

 teedev->cdev.kobj.parent = &teedev->dev.kobj;

TPM had the same mistake..

+void tee_device_unregister(struct tee_device *teedev)
+{
+	if (IS_ERR_OR_NULL(teedev))
+		return;

See for some general colour on IS_ERR_OR_NULL

https://www.mail-archive.com/linux-omap at vger.kernel.org/msg78030.html

IMHO, you should never, store an ERR pointer into long term storage,
so I wonder why this is like this...

+	if (teedev->flags & TEE_DEVICE_FLAG_REGISTERED) {
+		cdev_del(&teedev->cdev);
+		device_del(&teedev->dev);
+	}
+
+	tee_device_put(teedev);
+	wait_for_completion(&teedev->c_no_users);

Generally in a scheme like this we'd see open and release get/put the
underlying module handle to prevent driver removal while the char dev
is open. Otherwise module removal will hang here.

Jason